This is non-fatal by design. Ideally the messaging would be better and say "WARNING". But this is the only nonfatal warning in the entire program so there isn't code to vary this string.
Low priority, depending on code complexity may not be worth fixing.

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I'm trying to compile pdfium with "pdf_is_complete_lib = true" as a step towards using pdfium via FFI from Pharo Smalltalk.
But setting that option reported. ..
ERROR at //.gn:9:28: Build argument has no effect.
v8_extra_library_files = []
I'm a complete noob with ninja and gn (days old) and have no context to judge the importance of such a message or how to solve such an **ERROR**. The only context I have is the consistent convention I've known the last 20 years that "ERROR" means "WON'T BUILD", so I've lost hours over days googling how to solve this apparent show stopper. So I'd have to vote for correctness over convenience. There is always a first non-fatal error, before the next one.
Pragmatically, if a WARNING can't be added to the system, perhaps the at least the error message could be...
"Build argument has no effect. Really this is only a warning, not an error. Please proceed."
P.S. full `gn args` here... https://pastebin.com/rdxc8wvd